Production of polysaccharide-based polycarboxylates

ABSTRACT

A continuous process for the production of polycarboxylic acids or salts thereof from polysaccharides involving contacting a polysaccharide with an oxidizing agent selected from the group consisting of nitrogen dioxide, dinitrogen tetroxide, or an equilibrium mixture thereof in the absence of oxygen at a temperature above 80 °C. and at a pressure of from about 3 bar to about 15 bar absolute and for a tirne period of from about 1 to about 30 minutes to form a reducd gaseous product and a polysaccharide product wherein the primary alcohol groups of said polysaccharide product are at least partially converted to carboxyl groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the production ofpolycarboxylates by selective oxidation of polysaccharides with nitrogendioxide.

2. Statement of Related Art

The production of polycarboxylates by oxidative treatment ofpolysaccharides, for example cellulose, starch and dextrins, isdescribed in detail in the literature, cf. for example Houben-Weyl"Methoden der organischen Chemie", Thieme-Verlag, Stuttgart (1987), Vol.E 20, Makromolekulare Stoffe, Sub-chapter "Poly-saccharid-Derivate" byDr. K. Engelskirchen, loc. cit., pages 2042 et seq., more particularlypages 2124 et seg. (oxidation products of cellulose) and pages 2166 etseg. (oxidized starches). Information on production processes can alsobe found in the publication "Cellulose Chemistry and its Applications"(1983), John Wiley & Sons, Chichester, GB, cf. in particular T. P.Nevell, "Oxidation of Cellulose" (Chapter 10) and the literature citedtherein (loc. cit. pages 262 to 265).

Various oxidizing agents are used for the oxidation of polysaccharides,more particularly polyglucosans synthesized solely from glucose, andinclude for example (atmospheric) oxygen, hydrogen peroxide, sodiumhypochlorite or bromite, periodic acid and periodates, lead(IV) acetate,nitrogen dioxide and cerium(IV) salts. The oxidizing agents react verydifferently with the anhydroglucose units, cf. for example the formulaschemes in Houben-Weyl, loc. cit. page 2124. Thus, periodates andlead(IV) acetate promote C-C-opening of the anhydroglucose rings.So-called 2,3-dialdehyde cellulose is obtained from cellulose,dialdehyde starch being analogously obtained from starch. Wherecellulose is exposed to the action of nitrogen dioxide, the oxidation ofthe primary alcohol group to the carboxyl group is predominant. Theoxidizing agent, which is generally present in equilibrium withdinitrogen tetroxide, may be used in gaseous formed or as a solution inan inert organic solvent, cf. also Houben-Weyl, loc. cit. page 2125 andthe primary literature cited in this regard therein. Substantiallyselective oxidations of the primary alcohol group of the anhydroglucoseunits to the carboxyl group can also be carried out correspondinglystarting out from starch. Thus, the oxidation of starch with gaseousnitrogen dioxide or nitrogen dioxide dissolved in water or in variousorganic solvents at normal temperature and pressure is known from U.S.Pat. No. 2,472,590. In this case, the reaction temperature is relativelylow.

Under these conditions, the substantially complete conversion of theprimary alcohol groups of the polysaccharides into carboxyl groups isonly achieved after very long reaction times of, in some cases, severaldays. In addition, large amounts of nitrogen dioxide, based onpolysaccharide to be oxidized, are required in the known processes. Thelong reaction times are an obstacle to continuous operation and, inpractice, only allow discontinuous operation with relatively smallquantities of reactants. Another problem is the exothermic nature of thereaction.

The present invention seeks to improve the production of such oxidationproducts of polysaccharides in order to enable relatively largequantities to be economically produced and hence to secure theiravailability. This is because the polycarboxylates formed are potentialbuilders or co-builders for detergents and cleaning compositions. Thesame also applies to the salts of such polycarboxylates, moreparticularly their water-soluble salts. The use of oxidizedpolysaccharide compounds to boost the cleaning performance of detergentsand/or cleaning products has been known per se for decades and has beenrepeatedly investigated (cf. for example Dutch patents NL 69 883 and NL78 087). The replacement of phosphate-based builder systems by6-carboxycellulose treated with Lewis acids is described in U.S. Pat.No. 3,740,339 and in U.S. Pat. No. 3,790,561. The use of oxidizedpolysaccharide derivatives as a builder system for boosting detergency,particularly in laundry detergents, is also proposed in Dutch patentapplication NL 70/02 500. In this case, however, the derivatives inquestion are not derivatives selectively oxidized at the C₆ atom, butrather oxidation products formed by splitting of the anhydroglucoseunits between C₂ and C₃. Finally, European patent application EP 425 369describes surfactant-containing mixtures for the washing of laundrywhich contain a builder system of conventional phosphate compounds,zeolite and oxidation products of cellulose, starch or glucose sirup.There are no reproducible particulars in this document relating to thepreparation of the oxidized saccharide compounds described therein. Inaddition, stabilization of the polysaccharide oxidates initially formedby catalytic hydrogenation is said to be desirable.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a process flow diagram depicting a cascade of stirred tankreactors.

DESCRIPTION OF THE INVENTION

The teaching according to the present invention is based on thesurprising observation that polycarboxylates can be inexpensivelyobtained in high yields on a relatively large industrial scale by asimple process providing the oxidation reaction is carried outcontinuously with nitrogen dioxide/dinitrogen tetroxide at temperaturesabove 80° C. and over residence times of 1 to 30 minutes, preferably 5to 30 minutes and more preferably 10 to 30 minutes. At thesetemperatures, the oxidation reaction takes place sufficiently quicklyfor continuous processes. Surprisingly, the high temperatures do notadversely affect the degrees of oxidation of the polysaccharides or theproperties of their carboxyl derivatives, more particularly theirviscosity numbers. The expression "nitrogen dioxide/dinitrogentetroxide" stands for the equilibrium mixture of nitrogen dioxide andits dimer, dinitrogen tetroxide, which is present under the particularreaction conditions.

Accordingly, the present invention relates to a process for theproduction of polycarboxylic acids or salts thereof from polysaccharidesby oxidation with nitrogen dioxide/dinitrogen tetroxide with at leastpartial conversion of the primary alcohol groups of the polysaccharidesinto carboxyl groups and, optionally, at least partial neutralization ofthe carboxylic acid groups formed, the oxidation reaction being carriedout continuously at a temperature above 80° C. and over a residence timeof the reaction mixture in the reaction zone of 1 to 30 minutes,preferably 5 to 30 minutes and more preferably 10 to 30 minutes.Alternatively, the process may be carried out in the presence or absenceof oxygen as a co-oxidant.

The oxidation reaction is generally carried out at a temperature belowabout 160° C., preferably at temperatures of 100 to 140° C., morepreferably at temperatures of 100 to 130° C. and most preferably attemperatures of 110 to 120° C. Particularly good results are obtainedwhere the oxidation is carried out in a closed reaction system underpressures of 1 bar to 40 bar absolute and, more particularly, 3 bar to15 bar absolute, as measured at the reaction temperature. Where theoxidation is carried out in the absence of oxygen, nitrogendioxide/dinitrogen tetroxide is preferably used in such quantities that,where the equilibrium is theoretically completely displaced onto theside of the nitrogen dioxide, the nitrogen dioxide is present inquantities of 0.1 to 3 mole equivalents and, more particularly, 0.3 to 2mole equivalents, based on the monomer unit of the polysaccharidecontaining a primary alcohol group. If the oxidation is carried out inthe presence of oxygen, it is proposed that the nitrogendioxide/dinitrogen tetroxide be used in such quantities that, where theequilibrium is theoretically completely displaced onto the side of thenitrogen dioxide, the nitrogen dioxide is present in quantities of atmost 2 mole equivalents and, more particularly, 0.1 to 1 moleequivalent, based on the monomer unit of the polysaccharide containing aprimary alcohol group. The preferred pressures mentioned for thereaction system are established where nitrogen dioxide is used on itsown, i.e. in the absence of oxygen, under the reaction conditions and,where oxygen is used as co-oxidant, are established by the introductionunder pressure of gaseous oxygen or of a mixture of gaseous oxygen witha gas that is inert under the reaction conditions. Where oxygen is usedas a co-oxidant, a pressure of up to 10 bar is preferably established, apressure of 3 to 6 bar being preferred. The introduction of gaseousoxygen under pressure may be repeated several times, if desiredcontinuously, during the reaction. It is mentioned as a particularadvantage that the oxidation reaction can be controlled as a function oftemperature or pressure through the oxygen dosage. The addition ofoxygen is preferably controlled in such a way that the reactiontemperature remains in the above-mentioned range of 100 to 160° C.Throughout the oxidation reaction, which is best carried out withintensive mixing of the reactants, the reaction temperature cangenerally be maintained solely by the addition of oxygen, i.e. withoutexternal heating.

Suitable inert gases, i.e. gases which do not react under the particularprocess conditions required, include noble gases, such as helium orargon, and carbon dioxide, but especially nitrogen and also mixtures ofsuch gases. The oxygen content in the gas mixture is preferably from 1%by volume to 30% by volume and, more preferably, from 3% by volume to10% by volume. In one preferred embodiment of the process according tothe invention, oxygen is supplied by the introduction under pressure ofair, more particularly synthesis air.

Alternatively, the polysaccharides may be oxidized by the suspensionprocess in which they are used in the form of a suspension in a liquidwhich is substantially inert under the reaction conditions or theoxidation may be carried out as a gas/solid reaction with powder-formpolysaccharide.

Where oxidation is carried out by the suspension process, suitablesuspending agents for the polysaccharide are organic liquids which, onthe one hand, effectively dissolve nitrogen dioxide/dinitrogen tetroxideand oxygen and which, on the other hand, are largely inert to them underthe reaction conditions. Useful suspending agents are hydrocarbons and,above all, halogenated hydrocarbons, more particularly carbontetrachloride. The quantities of suspending agent used may be widelyvaried without any significant effect on the oxidation result. Ingeneral, the ratio by weight of suspending agent to polysaccharide is inthe range from 3:1 to 8:1. After passing through the reaction zone, thesuspending agent is separated from the oxidized polysaccharide by simplefiltration or centrifugation, optionally after removal of the excesspressure. The suspending agent may be returned to the mixing stageeither directly or if desired after working up. The oxidation product isoptionally washed with an organic solvent and/or water or mixturesthereof and dried. The drying step after washing with water may be leftout if the polycarboxylate obtained is to be subsequently processed towater-containing liquid or paste-form products.

The reaction may be carried out in standard reactors for gas/liquidreactions and also in continuous or on-line mixers. Thus, suitablereactors are tube reactors with static mixers arranged therein, cascadesof stirred tank reactors comprising at least two stages and stirredcolumns divided into at least two compartments. Where a cascade ofstirred tank reactors is used, temperature and pressure may beseparately established in each individual reactor. However, a columndivided into compartments may also be used as the reactor. Thisembodiment--which is equivalent to the cascade--may be more economicalunder certain conditions. By dividing up the column into compartments,individual sections can be separately cooled and heated as in the caseof the cascade. Not only the suspension liquid--as mentioned above--butalso the gas mixture containing nitrogen dioxide/dinitrogen tetroxidecan be recirculated.

The pipes through which the suspension flows are selected according tothe criteria that, on the one hand, the rate of flow of the suspensionshould be at least three times as high as the rate of descent of theparticles and, on the other hand, the specific pressure loss should beminimal or close to the minimum. Providing these basic principles areobserved, uninterrupted transport is possible throughout the entiresystem.

However, derivatization of the polysaccharides may also be carried outin the absence of a solvent or suspending agent. In this case, theoxidizing agents (nitrogen dioxide with--optionally--oxygen) actdirectly as gaseous reactants on the solid, intensively mixedsubstrates. In another embodiment of the invention, therefore,powder-form polysaccharide is used in the absence of a suspending agentor solvent and the reaction is carried out in a gas/solid reactor. Thereaction may be carried out in a loose-bed reactor, more particularly ina rotating tube furnace. The reaction may also be carried out in afluidized bed reactor. Alternatively, the reaction may be carried out ina continuous mixer or online mixer. A vibrating chute may also be used.

Where starch in particular is used as the polysaccharide (starch in itsnative state tending to agglomerate and to form channels in a fluidizedbed reactor), its flow behavior can be significantly improved byadditions of additives, including inter alia magnesium oxide, calciumfluoride, calcium phosphate or silica gel, more particularly highlydisperse pyrogenic silica, for example Aerosil® (a product of Degussa).The tendency of starch to agglomerate is greatly reduced even by theaddition of small quantities, for example 0.1 to 5% by weight and, moreparticularly, 0.25 to 1% by weight, of the additives. Correspondinglytreated starches show liquid-like behavior in regard to their mixingproperties. In this dry oxidation process, the reaction mixture can bedirectly taken up in water after the reaction, purified by washing withwater and filtration and isolated. A considerable proportion of thenitrogen oxides present at the end of the reaction zone can be removedby simple degassing processes. Thus, even a simple vacuum treatment ofthe reaction mixture leads to products with low nitrite and nitratecontents. It is also of advantage to recycle the gas phase at the end ofthe reaction zone after cooling and--where oxidation of thepolysaccharide is carried out in the absence of oxygen--oxidation of themain reduction product nitrogen monoxide to nitrogen dioxide. Thenitrogen oxides can thus be recirculated in a closed system, optionallytogether with oxygen and the inert gases.

The nature of the polysaccharide used is largely non-critical in theprocess according to the invention. The only requirement is that itshould contain carbohydrate units containing primary alcohol groups.Suitable polysaccharides are any native polyglucosans, more particularlystarch and/or cellulose, and other polysaccharides, for examplepolygalactomannans, such as guaran and carubin. The polysaccharides mayalso be used in chemically or physically modified form where they stillcontain oxidizable primary alcohol groups. For economic reasons,starches differing in their provenance, more particularly potato starch,wheat starch, cornstarch or tapioca starch, are preferred. Thepolysaccharide used preferably contains no more than 20% by weight and,more particularly, 4% by weight to 10% by weight of water.

The oxidation reaction of the process according to the invention iscarried out using the polyglucosans mentioned with--in particular--suchresidence times that, on a statistical average, at least 15 mole-%,preferably at least 25 mole-% and, more preferably, at least 35 mole-%to 40 mole-% of the oxidation product consists of oxidizedanhydroglucose units corresponding to formula I: ##STR1## whichcorresponds to a carboxyl group content of at least 4% by weight,another advantage being that no significant quantities of othersecondary oxidation products are present. The oxidation product thuspreferably has a content of oxidized anhydroglucose units correspondingto formula I of up to about 100 mole-% and, more particularly, in therange from about 70 mole-% to 95 mole-%.

Working up of the polycarboxylates obtained where the reaction iscarried out continuously, for example purification of the crude productsby washing with water, can be carried out by standard methods describedfor discontinuous processes.

Washing may be carried out discontinuously by suspending the crudepolycarboxylates--optionally after removal of the suspending agent usedby filtration, centrifugation or distillation--in water in a stirredvessel. The polysaccharide derivatives can be isolated from theresulting suspension by filtration or centrifugation.

Continuous working-up methods are of course particularly advantageousfor the claimed process. In these methods, the reaction product is takenup in water after leaving the reactor and separation of a suspendingagent and is delivered to continuously operated filters or centrifugeswhere it may optionally be rewashed with water.

The purified, water-containing polycarboxylates are dried eitherdirectly or after the removal of water by treatment with awater-miscible solvent. As described in the following, the purifiedmoist products may even be converted into the salt form and dissolved.

Thus, after the oxidation reaction and the optional working-up stepdescribed above, the carboxyl groups of the oxidation product may be atleast partly neutralized by treatment with a basic reagent, i.e.converted from the acid into the salt form. Preferably, theneutralization step is also carried out continuously. The neutralizingagent used is preferably an aqueous solution containing basic alkalimetal compound, more particularly alkali metal hydroxide, alkali metalcarbonate, alkali metal hydrogen carbonate, and/or ammonium hydroxideand/or organic base. An alkali metal hydroxide, more particularly sodiumhydroxide, sodium hydrogen carbonate or sodium carbonate, is preferablyused. Neutralization may also be carried out immediately after theoxidation reaction, for example by treating the crude product withgaseous ammonia. Salt formation may even be carried out under reducingconditions, for example using sodium borohydride. The neutralizing agentis preferably used in such quantities that all the carboxyl groups ofthe oxidation product are converted into the salt form. The oxidationproduct may be added to the neutralizing agent or the neutralizing agentmay be added to the oxidation product. Salt formation may even becarried out in the practical application or subsequent processing of thepolycarboxylates present in acid form, for example in the production oruse of detergents or cleaning products, by typical alkaline constituentsof such products.

The polycarboxylates produced by the process according to the inventionare preferably used as builders or co-builders in detergents or cleaningcompositions. In such detergents or cleaning compositions, they arepreferably used as co-builders in quantities of 0.5% by weight to 10% byweight and, more particularly, in quantities of 2% by weight to 7% byweight, based on the total weight of the detergent or cleaningcomposition containing inorganic water-insoluble primary builders. In aparticularly preferred embodiment, they are used in detergents orcleaning compositions containing zeolite NaA of the type described, forexample, in German patent DE 24 12 837 in connection with laundrydetergents as primary builder and polycarboxylic acids produced inaccordance with the invention or salts thereof in quantity ratios of 2:1to 5:1. The composition of the detergent and/or cleaning compositionsmay otherwise be selected as required within the limits of knownformulations.

In addition, the polycarboxylic acids produced in accordance with theinvention are suitable for use in dishwashing detergents. In addition tosurfactants, machine dishwashing detergents of the latest phosphate-freegeneration contain oxygen-based oxidizing agents and a relativelycomplex builder combination of alkali metal carbonates and organiccomplexing agents. The organic complexing agents hitherto used includeorganophosphonic acids, hydroxycarboxylic acids, aminocarboxylic acidsand, in particular, polymeric polycarboxylic acids which are often usedin admixture with low molecular weight polyhydroxypolycarboxylic acids,for example citric acid, or water-soluble salts thereof. In addition,alkali metal silicate is generally present as a further buildercomponent. Dishwashing detergents with this composition often have thedisadvantage of forming lime deposits both in the dishwashing machineused and, in particular, on the washed contents thereof, particularlywhere relatively hard water is used. The coatings formed by thedishwashing detergent in conjunction with the hardness constituents ofthe water can be inhibited by the use of a builder combinationcontaining certain oxidation products of polyglucosans. In anotheradvantageous embodiment of the invention, therefore, the polycarboxylicacids produced by the process described above or salts thereof are usedas builders or as part of a builder composition in dishwashingdetergents intended in particular for dishwashing machines.

In addition, the polycarboxylates produced by the process according tothe invention are eminently for suitable for use as incrustationinhibitors against lime deposits in water-carrying pipes and apparatus,for example in cooling water systems.

The polycarboxylates may also be used with advantage as auxiliaries inceramic compositions for improving their plastic forming properties.

The claimed process is illustrated by the following Examples relating tothe NO₂ oxidation of starch by the suspension process in conjunctionwith the accompanying drawing.

EXAMPLES

Example 1

Example 1 describes the oxidation of starch in the cascade of stirredtank reactors illustrated in FIG. 1. The cascade consists of fourindividual stirred tank reactors which are designed in such a way thatthe useful volume of the following tank reactor is twice that of thepreceding tank reactor. The overall useful volume is of the order of 250l.

Each tank reactor is equipped with a separately driven stirrer systemwith speed control and with a separately controllable jacket cooling andheating system.

120 kg/h of starch with a water content of around 4% by weight weredelivered to the mixing vessel 5 from tank 1 via a weighing belt 2 while956 kg/h of carbon tetrachloride were continuously delivered to themixing vessel 5 from tank 2 via a metering pump 4. The resulting starchsuspension was continuously removed in a quantity of around 1076 kg/hthrough another metering pump 6 and introduced via a heating zone 8 intothe cascade consisting of the stirred tank reactors 9 and 12.

Before entering the heating zone, 65.4 kg/h of liquid dinitrogentetroxide were added to the suspension from the vessel 7.

At the end of the heating zone, the reaction mixture had reached atemperature of around 90° C. It then entered the stirred tank reactors 9to 12 in which it was heated to temperatures of around 95° C. (stirredtank reactor 9), 105° C. (stirred tank reactor 10), 115° C. (stirredtank reactor 11) and 125° C. (stirred tank reactor 12).

A pressure of around 15 bar was established in the cascade of stirredtank reactors.

After leaving the stirred tank reactor 12, the reaction mixture enteredthe cooling zone 13 in which it was cooled to around 65 to 70° C. It wasthen relieved of pressure through the valve 14. The nitrous gasesescaping were oxidized in the presence of air and returned to thestorage vessel 7 for dinitrogen tetroxide.

The reaction mixture was then delivered to a continuously operatingcentrifuge 15. Most of the carbon tetrachloride was removed bycentrifugation and, after extraction with water, was returned to thestorage tank 2.

The solvent adhering to the solid was evaporated off in vacuo, condensedand optionally returned to the storage tank 2.

The crude carboxyl starch was washed with water and then dried to aresidual moisture content of around 6% by weight.

A white product with an acid value of around 290, corresponding to anaverage content of around 0.9 carboxyl groups per anhydroglucose unit,was obtained.

To determine the acid value, around 0.5 g to 0.75 g of the carboxylstarch was suspended in 50 ml of demineralized water. 10 ml of 0.5 Nalcoholic potassium hydroxide solution were added to the resultingsuspension which was then stirred for 30 minutes at room temperature,the starch derivative passing into solution. Excess potassium hydroxidewas back-titrated with 0.5 N aqueous hydrochloric acid againstphenolphthalein as indicator.

Example 2

Example 1 was repeated with the difference that 32.7 kg/h of liquefieddinitrogen tetroxide were added to the starch suspension before itentered the heating zone and a pressure of around 6 bar absolute wasestablished with nitrogen in the cascade of stirred tank reactors andwas kept substantially constant by the subsequent introduction ofoxygen.

Working up of the reaction mixture produced a carboxyl starch with anacid value of around 320 corresponding to a content of around onecarboxyl group per anhydroglucose unit.

Example 3

Example 2 was repeated with the difference that the cascade of stirredtank reactors was replaced by a stirred column divided into 12compartments.

With a length of 2.5 m and a diameter of 0.4 m, this reactor had auseful volume of around 295 l. In terms of stage efficiency, this columncorresponded to a cascade arrangement with 6 to 7 individual tankreactors of equal size.

The column was blanketed with compressed air up to a pressure of around5 bar (synthesis air). This pressure was kept constant by subsequentintroduction of oxygen.

The temperature of the reaction mixture was around 95° C. at the end ofthe heating zone, around 105° C. in the 1st third of the column, around120° C. in the 2nd third and around 130° C. in the 3rd third of thecolumn. The average residence time was around 25 minutes.

Working up of the reaction mixture in the same way as described inExample 1 gave a carboxyl starch with an acid value of 310 correspondingto a content of around one carboxyl group per anhydroglucose unit.

    ______________________________________    List of reference numerals    ______________________________________    1           Storage tank for starch    2           Storage tank for carbon tetrachloride    3           Weighing belt    4           Metering pump    5           Mixing vessel    6           Pump    7           Storage vessel for dinitrogen tetroxide    8           Heating zone    9-12        Stirred tank reactors    13          Cooling zone    14          Pressure reduction    15          Centrifuge    ______________________________________

What is claimed is:
 1. A continuous process for the production ofpolycarboxylic acids or salts thereof from polysaccharides whichcomprises contacting a polysaccharide with an oxidizing agent selectedfrom the group consisting of nitrogen dioxide, dinitrogen tetroxide, oran equilibrium mixture thereof in the presence or absence of oxygen at atemperature above 80° C. and at a pressure of from about 3 bar to about15 bar absolute and for a time period of from about 1 to about 30minutes to form a reduced gaseous product and a polysaccharide productwherein the primary alcohol groups of said polysaccharide product are atleast partially converted to carboxyl groups.
 2. The process of claim 1wherein the reaction is carried out in the presence of oxygen.
 3. Theprocess of claim 1 wherein the reaction is carried out at a temperatureof from about 100° C. to about 140° C.
 4. The process of claim 3 whereinsaid temperature is from about 110° C. to about 120° C.
 5. The processof claim 1 wherein said reaction time is from about 5 to about 30minutes.
 6. The process of claim 5 where said time is from about 10 toabout 30 minutes.
 7. The process of claim 1 wherein said oxidizing agentis nitrogen dioxide present in an amount equal to from about 0.1 toabout 3 mole equivalents based on the monomer unit of the polysaccharidecontaining a primary alcohol group.
 8. The process of claim 7 whereinthe amount of said nitrogen dioxide is from about 0.3 to about 2 moleequivalents based on the monomer unit of the polysaccharide containing aprimary alcohol group.
 9. The process of claim 1 further comprising thestep of oxidizing the nitrogen monoxide in said gaseous product tonitrogen dioxide and recycling it to the polysaccharide oxidationreaction.
 10. The process of claim 2 wherein said oxidizing agent isnitrogen dioxide present in at least 2 mole equivalents based on themonomer unit of the polysaccharide containing a primary alcohol group.11. The process of claim 10 wherein the amount of said nitrogen dioxideis from about 0.1 to about 1 mole equivalents based on the monomer unitof the polysaccharide containing a primary alcohol group.
 12. Theprocess of claim 2 wherein the pressure is kept constant by introductionof oxygen.
 13. The process of claim 2 wherein the pressure is keptconstant by introduction of a mixture comprised of oxygen and an inertgas selected from the group consisting of helium, argon, carbon dioxide,nitrogen and mixtures thereof wherein the oxygen content of said mixtureis from about 1% to about 30% by volume.
 14. The process of claim 2wherein the pressure is kept constant by introduction of a mixturecomprised of air and an inert gas selected from the group consisting ofhelium, argon, carbon dioxide, nitrogen and mixtures thereof wherein theoxygen content of said mixture is from about 1% to about 30% by volume.15. The process of claim 1 wherein said polysaccharide is polygucosanand the reaction is carried for a time sufficient to produce a productcomprised of at least 15 mole-% oxidation product which is comprised ofup to 100 mole-% of oxidized anhydroglucose units of the formula I:##STR2##
 16. The process of claim 15 wherein the amount of said oxidizedanhydroglucose units is from about 70 mole-% to about 95 mole-%.
 17. Theprocess of claim 1 wherein said polysaccharide is suspended in a liquidwhich is substantially inert under the reaction conditions.
 18. Theprocess of claim 17 wherein said liquid is a halogenated hydrocarbon.19. The process of claim 18 wherein said halogenated hydrocarbon iscarbon tetrachloride.
 20. The process of claim 1 wherein said process iscarried out in a tube reactor containing a static mixer, a cascade ofstirred tank reactors comprising at least two stages, a continuous mixeror in a stirred column divided into at least two compartments, aloose-bed reactor, a rotating tube furnace, a fluidized bed reactor, acontinuous mixer or online mixer.
 21. The process of claim 1 whereinsaid polysaccharide is derived from potato starch, wheat starch,cornstarch, tapioca starch, cellulose or mixtures thereof.
 22. Theprocess of claim 17 further comprising the steps of removing thepolysaccharide product from said liquid and washing said polysaccharideproduct with water.
 23. The process of claim 1 further comprising thestep of at least partially neutralizing said carboxyl groups by reactionwith an alkali metal hydroxide, alkali metal carbonate, alkali metalhydrogen carbonate, ammonium hydroxide, organic base or a combinationthereof.
 24. The process of claim 2 wherein said polysaccharide ispolyglucosan and the reaction is carried for a time sufficient toproduce a product comprised of at least 15 mole-% oxidation productwhich is comprised of up to 100 mole-% of oxidized anhydroglucose unitsof the formula I:
 25. The process of claim 24 wherein the amount of saidoxidized anhydroglucose units is from about 70 mole-% to about 95mole-%.
 26. The process of claim 1 wherein said polysaccharide issuspended in a liquid which is substantially inert under the reactionconditions.
 27. The process of claim 26 wherein said liquid is ahalogenated hydrocarbon.
 28. The process of claim 27 wherein saidhalogenated hydrocarbon is carbon tetrachloride.
 29. The process ofclaim 2 wherein said process is carried out in a tube reactor containinga static mixer, a cascade of stirred tank reactors comprising at leasttwo stages, a continuous mixer or in a stirred column divided into atleast two compartments, a loose-bed reactor, a rotating tube furnace, afluidized bed reactor, a continuous mixer or online mixer.
 30. Theprocess of claim 2 wherein said polysaccharide is derived from potatostarch, wheat starch, cornstarch, tapioca starch, cellulose or mixturesthereof.
 31. The process of claim 26 further comprising the steps ofremoving the polysaccharide product from said liquid and washing saidpolysaccharide product with water.
 32. The process of claim 2 furthercomprising the step of at least partially neutralizing said carboxylgroups by reaction with an alkali metal hydroxide, alkali metalcarbonate, alkali metal hydrogen carbonate, ammonium hydroxide, organicbase or a combination thereof.